The XOpenDevice
request makes an input device accessible to a client through input extension
protocol requests. If successful, it returns a pointer to an XDevice structure.

The XCloseDevice request makes an input device inaccessible to a client
through input extension protocol requests. Before terminating, and client
that has opened input devices through the input extension should close
them via CloseDevice.

When a client makes an XCloseDevice request, any active
grabs that the client has on the device are released. Any event selections
that the client has are deleted, as well as any passive grabs. If the requesting
client is the last client accessing the device, the server may disable
all access by X to the device.

The classes field is a pointer to an array of XInputClassInfo structures.
Each element of this array contains an event type base for a class of input
supported by the specified device. The num_classes field indicates the
number of elements in the classes array.

The input_class field identifies one class of input supported by the device.
Defined types include KeyClass, ButtonClass, ValuatorClass, ProximityClass,
FeedbackClass, FocusClass, and OtherClass. The event_type_base identifies
the event type of the first event in that class.

The information contained
in the XInputClassInfo structure is used by macros to obtain the event
classes that clients use in making XSelectExtensionEvent requests. Currently
defined macros include DeviceKeyPress, DeviceKeyRelease, DeviceButtonPress,
DeviceButtonRelese, DeviceMotionNotify, DeviceFocusIn, DeviceFocusOut,
ProximityIn, ProximityOut, DeviceStateNotify, DeviceMappiingNotify, ChangeDeviceNotify,
DevicePointerMotionHint, DeviceButton1Motion, DeviceButton2Motion, DeviceButton3Motion,
DeviceButton4Motion, DeviceButton5Motion, DeviceButtonMotion, DeviceOwnerGrabButton,
DeviceButtonPressGrab, and NoExtensionEvent.

To obtain the proper event
class for a particular device, one of the above macros is invoked using
the XDevice structure for that device. For example,

DeviceKeyPress (*device, type, eventclass);

returns the DeviceKeyPress event type and the eventclass for DeviceKeyPress
events from the specified device.

This eventclass can then be used in an
XSelectExtensionEvent request to ask the server to send DeviceKeyPress
events from this device. When a selected event is received via XNextEvent,
the type can be used for comparison with the type in the event.

An invalid device was specified. The specified device does not exist, or
is the X keyboard or X pointer. This error may also occur if some other
client has caused the specified device to become the X keyboard or X pointer
device via the XChangeKeyboardDevice or XChangePointerDevice requests.